High Cr steel like stainless steel has been conventionally produced using ferrochromium as raw material. In view of saving energy and securing low production cost, smelting reduction process has been recently considered as being potentially attractive. This process produces high Cr molten metal directly using Cr raw ores (explanation will refer to "Cr ore" as an example hereinafter). In the smelting reduction method, Cr ores, carbonaceous materials and others are supplied into a reduction furnace for directly obtaining high Cr molten metal.
For the smelting reduction method, there have been several proposals hitherto. One, proposal blows respectively O.sub.2 from bottom tuyeres and N.sub.2 from side blowing tuyeres at the same time as blowing O.sub.2 from a top lance. Another proposal blows respectively O.sub.2 from the bottom tuyeres and O.sub.2 and N.sub.2 from the side blowing tuyeres at the same time as blowing O.sub.2 from the top lance. The latter is disclosed in, for example, Japanese Patent Application Laid-Open No. 61-279,608 (1986).
However, each of the above mentioned methods has many problems in that the Cr reduction speed is low and treatments take considerable time. With respect to the background of the prior art, the following drawbacks are known.
1. The workers in the art believed that reduction of Cr ore progresses by action of C in carbonaceous materials after Cr ore had been molten in the slag, and the melting of Cr ore is assumed to determine Cr reduction. Therefore, important technical interests for shortening the treating time were focused on specifying of the slag composition. But Cr ore is inherently less to be molten, and it is limited to speed up the reduction by accelerating the melting of Cr ore.
2. For accelerating the melting speed of Cr ore in the slag and the reduction speed, it has been considered to make a post combustion of CO gas in the furnace and utilize the heat thereby. An existing method has been adopted, which blew O.sub.2 for the post combustion from a furnace upper part. If the post combustion ratio was increased, the temperature of an exhaust gas went up, but there was no technique which efficiently transmitted a sensitive heat of the exhaust gas to the molten metal, and as a result, heat transfer efficiency was lowered and the exhaust gas at high temperature was removed inevitably. The heated exhaust gas considerably damaged the wall refractories and those of an exhaust gas feed. Therefore, in general it has been considered that the post secondary combustion ratio could not be heightened very much.
For dealing with the above stated problems of the prior art, the inventors made studies on the mechanisms of the smelting reduction and the decarburization, and the actual measures therefor, and subsequently they found following facts.
(1) As having been said above, it was assumed that Cr ore was reduced by the carbonaceous materials staying in the slag after Cr ore was molten in the slag, but it has been found that almost all of the actual reductions were made by actions of C in the molten metal beneath the slag as the reducing material. Therefore, the reduction speed was determined by contacting of the molten metal to Cr ore heated at the high temperature not by melting of Cr ore into the slag, so that the reduction speed could be effectively heightened by positively contacting of the molten metal to the ores.
(2) It was a basic concept in the prior art that the post combustion which is combustion of CO.sub.2 generated by reduction of carbon in the ore increased in view of the technical limit with respect to increasing of the heat transfer efficiency and consumption of the refractories. If O.sub.2 was blown so that the post combustion was mainly caused in the slag to forcibly agitate the slag, the heat transfer efficiency could be heightened-effectively. Thus, by the high post combustion and the high heat transfer efficiency, the temperatures of the slag and Cr ore in the slag were heightened, and the reduction speed of Cr ore by C (in the molten metal) expressed by the following formula, may be raised effectively EQU Cr.sub.2 O.sub.3 +3C=2Cr+3CO.
(3) In the prior art foregoing technique sometimes carried out the bottom blowing of O.sub.2 in a certain period or in a full term, but such blowing was harmful to the post combustion. That is, if O.sub.2 was blown from the bottom, CO gas was generated in the molten metal and agitated the molten metal compulsively, and splashes of the molten metal reached the region of the post combustion, and since C reacted with O.sub.2 or CO.sub.2 and generated CO, the post combustion was hindered. Therefore the bottom blowing had to be avoided, irrespective of a part and the full term of the reduction period.
Cr raw ores are very fine in grain diameters, and ordinarily around 90% contain those having grain diameters of not more than 1 mm. Therefore, when Cr raw ores such as powders are charged into the reduction furnace of the converter type from a top part, the powder ores fly outside of the furnace and lose up to 30% are lost.
For avoiding the flying losses, an injection charging may be suggested, but special facilities are required independently, and the transporting pipes are easily injured by the hard Cr ores. Thus, such measures could not be adopted actually.
In view of these circumstances, Cr raw ores are formed into pellets or briquets which results in high production costs. If the ores are agglomerated, the specific surface areas of the ores are made small so that a pre-heating time is made long and the reduction speed is lowered to lengthen the treating time.
The inventors made studies also on those problems with respect to practical measures, and as a result it was found that the probability that the grains of the Cr ores are exposed in the up-flowing gas, was decreased by heightening the charging speed of the ores and the flying loss was lowered, and especially if the charging speed of Cr raw ores is determined to be more than 4 kg/min-molten metal ton per pure Cr, the flying loss could be largely decreased.